Method and device for producing fine powder by atomizing molten material with gases

ABSTRACT

A method and nozzle for producing fine powder by atomizing molten material with gas are provided. Molten material, in the form of a film, flows out of a molten material nozzle that has an essentially rectangular discharge cross-sectional area. Thereafter, the molten material, together with an atomizing gas, flows through an initially converging and then diverging gas nozzle that is in the form of a linear Laval nozzle, has an essentially rectangular cross-sectional area, and through which flow is laminar. Laminar accelerated gas flow stabilizes and simultaneously stretches the film of molten material in the converging portion of the Laval nozzle until the film of molten material, after passing the narrowest cross-sectional area of the Laval nozzle, is uniformly atomized over its entire length.

BACKGROUND OF THE INVENTION

The present invention relates to a method and nozzle for producing finepowder, preferably having a spherical physical appearance, by atomizingmolten material with gases such as known for EP-A-0444-767.

To produce metal powders, gas atomization techniques are knownthroughout the industry. Different nozzle constructions are utilized,all of which have in common that a pressurized atomization gas escapesfrom one or more gas nozzles and, as a turbulent stream, approaches atan angle molten material flowing out of a molten material nozzle andatomizes such molten material. An overview of various nozzleconstructions is provided, for example, by A. J. Yule and J. J. Dunkley“Atomization of Melts”, Oxford, 1994, pages 165 to 189. On its way tothe molten material, the gas loses a large portion of its energy. Withatomization gas pressures up to about 35 bar, relatively coarse metalpowder having average granular diameters d₅₀ in the atomization state ofabout 50 μm and greater result. The thus produced powders generally havea broad granular size distribution because the atomization pulse issubjected to great deviations due to the turbulence. J. Ting, et al., “Anovel high pressure gas atomizing nozzle for liquid metal atomization”,Adv. Powder Metallurgy and Particulate Materials, 1996, pages 97 to 108,discloses special high pressure nozzles having operating pressures of upto 100 bar, which at a very high gas consumption can produce averagegranular sizes of about 20 μm. All known methods having turbulent gasflow are unsuitable for the direct production of fine powders havingaverage granular diameters d₅₀ of about 10 μm.

DE 33 11 343 A1 discloses a method of producing fine metal powders aswell as a device for carrying out the method, and proposes the use oflaminar gas streams in a concentric Laval nozzle having preheatedatomizing gas. The molten material nozzle is positioned in such a waythat it is disposed in the converging portion of the Laval nozzle, i.e.,that the molten material nozzle extends into the Laval nozzle. The flowin the upper portion of the Laval nozzle is laminar. In contrast tomethods having turbulent gas flows, finer powder with a narrowergranular size distribution, accompanied by relatively low specific gasconsumption, result, as illustrated, for example, in FIG. 2 of thepublication of G. Schulz, “Laminar sonic and supersonic gas flowatomization” PM²TEC '96, World Congress On Powder Metallurgy AndParticulate Materials, U.S.A., 1996, pages 1 to 12. The specific gasconsumption for the production of a steel powder having an averagegranular diameter of 10 μm is approximately 7 to 8 Nm³Ar/kgcorresponding to about 12.5 kg to 14.2 kgAr/kg steel.

DE 35 33 964 C1 discloses a method and an apparatus for producing veryfine powders in spherical form, according to which the atomizing gas isintroduced via a radially symmetrical, heatable gas hopper into theLaval nozzle, whereby the metal exiting the molten material nozzle,which is placed within this gas hopper, is overheated or heated by heattransfer via radiation, which originates from the heated gas hopper.

DE 37 37 130 A1 similarly discloses a method and an apparatus forproducing very fine powders, according to which the underpressureresulting from the gas flowing in the Laval nozzle is utilized to drawin molten material from a separate molten material device. Here also aradially symmetrical nozzle system having a molten material nozzleplaced within the Laval nozzle is involved.

From the publication of G. Schulz, “Laminar sonic and supersonic gasflow atomization—The NANOVAL—Process”, Adv. Powder Metall. & ParticulateMatter. (1996), 1, pages 43-54, it is furthermore known that for theproduction of fine metal powder it is necessary to keep the mass flowexiting the radially symmetrical nozzle small if fine powder is to beproduced. Indicated here are 12 to 30 kg/h and nozzles with moltenmaterial nozzle diameters of 1 mm or less.

Common to all of the previously known methods is that these have serioustechnical and economical drawbacks. For example, the heretofore utilizedconcentric or radially symmetrical nozzle systems having molten materialnozzle diameters of 1 mm or less, are, due to the type of construction,particularly susceptible to mechanical clogging due to foreign particlesor gas bubbles that are carried along. In addition, due to a givenunfavorable ratio of outer molten material nozzle surfaces to the moltenmaterial volume, great heat losses occur that can effect an undesiredcongealing of the molten material nozzles and then, as is also the casewith the mechanical clogging, result in a termination of the atomizationand longer down times. Furthermore, the production capacity that up tonow could be achieved is low, and the specific gas consumption is high.During the production of fine powders, the production capacity and thespecific gas consumption are very decisive in determining themanufacturing costs. There is therefore a need for an atomizing methodthat is characterized by low gas consumption and high productioncapacity.

Taking into account this state of the art, the object of the inventionis to improve a method of the aforementioned general type, whileavoiding the described drawbacks, in such a way that an economicalproduction of fine, gas-atomized powder is possible. Furthermore, downtimes due to clogging from impure molten material, and from congealingdue to heat losses, are to be avoided. In particular, it should bepossible to finely and uniformly atomize metallic, metallic alloy, salt,salt mixture, or also polymeric molten material on a large scale, in aneconomical manner, and in particular, however, with a low gasconsumption and a high molten material throughput. Furthermore, themolten material nozzle should be as stable as possible relative tomechanical clogging from impure molten material as well as relative tocongealing.

SUMMARY OF THE INVENTION

The object is inventively realized in that the molten material flows outof a molten material nozzle having an essentially rectangularcross-sectional area in the form of a film, and subsequently, togetherwith an atomizing gas, issues through an initially converging and thendiverging gas nozzle that is in the form of a linear Laval nozzle, hasan essentially rectangular cross-sectional area, and through which flowis laminar, whereby the laminar accelerated gas flow stabilizes andsimultaneously stretches the film of molten material in the convergingportion of the Laval nozzle until the film of molten material, afterpassing the narrowest cross-sectional area, is uniformly atomized overits entire length.

Surprisingly, it is possible to stabilize the film of molten material,which is primarily issuing from the essentially rectangular moltenmaterial nozzle, and that would be unstable due to its large surfacearea by virtue of free discharge, by the introduction into theaccelerated gas stream in the converging portion of the similarlyessentially rectangular Laval nozzle. In so doing, an extremelyfavorable relationship of powder molten material nozzle surface to themolten material volume is achieved, so that clogging due to congealingis precluded. Furthermore, individual foreign particles in impure moltenmaterial can in the most unfavorable situation affect only a smallportion of the cross-sectional area of the molten material nozzle, sothat the atomizing process is also not terminated under such conditions.Below the narrowest cross-sectional area of the Laval nozzle, the filmof molten material is uniformly atomized with high specific pulse to afine powder that preferably has a spherical physical appearance.

Pursuant to a further advantageous proposal of the invention, the ratioof the pressure above the Laval nozzle and below the Laval nozzlecorresponds at least to the critical pressure ratio of the atomizing gasthat is utilized, so that the gas reaches the speed of sound in thenarrowest cross-sectional area of the Laval nozzle. The pressure ratiois advantageously >2, preferably >10.

Pursuant to a further advantageous proposal of the invention, theatomizing gas is preheated. Pursuant to a further advantageous proposalthe molten material issuing from the molten material nozzle is heatedvia radiation. The preheating of the atomizing gas, and the heating-upof the molten material via radiation, are, however, not necessaryprerequisites for being able to carry out the method. A preheating ofthe atomizing gas, and a heating-up of a molten material issuing fromthe molten material nozzle via radiation, are preferably dispensed with,as a result of which on the one hand the capital outlay for equipment isconsiderably reduced, and on the other hand energy is saved.

Pursuant to a further advantageous proposal of the invention, impuremolten material is also atomized by the molten material nozzle. Metals,metal alloys, salts, salt mixtures, or synthetic materials that can bemelted, such as polymers, are advantageously utilized as molten materialthat is to be atomized.

Pursuant to a further advantageous proposal of the invention, the moltenmaterial that is to be atomized does not react with the atomizing gas,in other words, is inert relative to the gas. If the material that is tobe atomized does not react with the atomizing gas, in other words isinert relative to the gas, spherical particles form from the moltenmaterial droplets under the influence of surface tension. Pursuant to afurther proposal of the invention, the molten material that is to beatomized reacts entirely or partially with the atomizing gas. If thematerial that is to be atomized, in other words the molten material,reacts entirely or partially with the atomizing gas, reaction productsare thereby formed that can prevent the molten material droplets fromforming into spheres, so that irregularly formed powder particles areformed. If a substrate is advantageously introduced into the particlestream at a distance at which the particles are at least still partiallyliquid, the direct production of a semi-finished product is possible, aso-called atomizing compaction.

Pursuant to the method, the ratio of the cross-sectional surfaces of themolten material nozzle discharge to the narrowest cross-sectional areaof the Laval nozzle with linear systems is always greater than withradially symmetrical nozzles. Since the throughput quantities of gas andmetal and the like under otherwise identical conditions are proportionalto the corresponding nozzle cross-sectional areas, pursuant to themethod there results linear systems having lower specific gasconsumptions. The savings increases with the length of the nozzlesystem. Due to the proportionality of molten material nozzlecross-sectional area and molten material throughput, by adaptation ofthe nozzle length every desired production capacity can be set in asimple manner. The characteristic properties of metal powder, such asgranular size, width of the granular size distribution, and granularshape, thereby remain unchanged, in contrast to which pursuant to themethod the specific gas consumption drops.

As a device for carrying out the method of the invention a nozzle foratomizing molten material is inventively proposed that has a moltenmaterial nozzle and a gas nozzle disposed below the discharge thereof inthe direction of flow, with the inventive nozzle being characterized inthat the molten material nozzle has an essentially rectangular dischargecross-sectional area, in that the gas nozzle also has an essentiallyrectangular cross-sectional area in the form of a linear Laval nozzle,and in that the gas nozzle generates an initially converging, laminaraccelerated gas flow that stabilizes and simultaneously stretches thefilm of molten material until after passing the narrowestcross-sectional area in the diverging portion of the gas nozzle the filmof molten material is uniformly atomized over its entire length. As aconsequence of the inventive nozzle with an essentially rectangularcross-sectional area, in other words, a rectangular or substantiallyrectangular cross-sectional area, the cross-sectional area can, byaltering the length of the rectangle, be adapted in such a way thatevery desired molten material throughput can be achieved and in this waya high production capacity results.

Pursuant to one advantageous proposal of the invention, the dischargecross-sectional area of the molten material and/or the Laval nozzles aremodified in such a way that the two short sides of the rectangle of thenozzle cross-sectional area are replaced by semi circular arcs having adiameter corresponding to the length of the short sides, so that asubstantially rectangular cross-sectional area is provided.

Pursuant to a further particularly advantageous proposal of theinvention, the ratio of the long side of the rectangle and the shortside of the rectangle of the discharge cross-sectional area of themolten material and/or of the Laval nozzles is at least >1preferably >2, and especially preferably >10. Pursuant to a furtheradvantageous proposal of the invention, the length of the linear Lavalnozzle in the narrowest cross-sectional area is greater than the lengthof the molten material nozzle. Advantageously, the ratio of the width ofthe Laval nozzle to the width of the molten material nozzle >1 and <100,preferably <10.

Pursuant to a further particularly advantageous proposal of theinvention, the molten material throughput is adapted to the desiredproduction capacity by lengthening the long side of the molten materialnozzle and corresponding lengthening of the long side of the Lavalnozzle by the same amount, without thereby altering the granular size ofthe powder that is to be produced or increasing the specific gasconsumption.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

Further details, features and advantageous of the present invention canbe seen from the following description of the pertaining drawings, inwhich one preferred specific embodiment of the invention isschematically illustrated. Shown are:

FIG. 1 a schematic perspective view showing the inventive atomizationprinciple, and

FIG. 2 a projection of the discharged cross-sectional area of the moltenmaterial nozzle onto the narrowest cross-sectional area of the Lavalnozzle.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1, in a schematic perspective view, shows the method and apparatusof the atomizing principle. A gas chamber 1 having a high pressure p₁ isseparated from a gas chamber 2 having low pressure p₂ by an initiallyconverging and then diverging gas nozzle 3, which has an essentiallyrectangular cross-sectional configuration and is in the form of a linearLaval nozzle. The pressure ratio p₁/p₂ above the Laval nozzle and belowthe Laval nozzle corresponds to at least the critical pressure ratio ofthe atomizing gas that is utilized, so that in the narrowestcross-sectional area of the Laval nozzle 3 the gas reaches the speed ofsound. The greater is the atomizing gas pressure p₁, the finer is theresulting powder. The molten material 5 flows out of the film-formingmolten material nozzle 4, which has an essentially rectangular dischargecross-sectional area, in the form of a film. In this connection, themolten material nozzle 4 is embodied as a casting distributor or meltingcrucible. The molten material 5 of the material that is to be atomizedis produced and made available via known techniques. The discharge ofthe molten material nozzle 4 is positioned above the Laval nozzle 3 andis oriented parallel thereto. As a consequence of the pressuredifferential the atomizing gas flows from the gas chamber 1 into the gaschamber 2. In the converging portion of the Laval nozzle 3 the gas isaccelerated in a laminar flow in the narrowest cross-sectional areauntil the speed of sound is reached. The gas always flows with a greaterspeed than does the molten material 5, and stabilizes, stretches andaccelerates the film 6 of molten material. Below the narrowestcross-sectional area of the Laval nozzle 3, the thin film 6 of moltenmaterial is finally atomized with high specific pulse over its entirelength and uniformly to a fine particle stream 7 of droplets of moltenmaterial that then give off their heat and solidify to a fine powder.The stable, thin film 6 of molten material is the prerequisite for theproduction of particularly fine powder having an average granulardiameter d₅₀ of about 10.

FIG. 2 shows a projection of the discharge area 8 of the molten materialnozzle 4 upon the narrowest cross-sectional area 9 of the Laval nozzle3. The discharge cross-sectional area 8 of the molten material nozzle 4,and the narrowest cross-sectional area 9 of the Laval nozzle 3, areprovided at the two short sides b_(sd)b_(ld) with circular arcs having adiameter corresponding to the lengths of the short sides b_(sd)b_(ld) sothat in each case a substantially rectangular cross-sectional area isprovided. The ratio, which is not actual size, shown in FIG. 2 of thelong rectangular sides a_(sd)a_(ld) of the discharge cross-sectionalarea 8 of the molten material nozzle 4 and of the narrowestcross-sectional area 9 of the Laval nozzle 3, and the short rectangularsides b_(sd)b_(ld) of the discharge cross-sectional area 8 of the moltenmaterial nozzle 4 and of the narrowest cross-sectional area 9 of theLaval nozzle 3 is >10. The length a_(ld) of the narrowestcross-sectional area 9 of the Laval nozzle 3 is greater than the lengtha_(sd) in the discharge cross-sectional area 8 of the molten materialnozzle 4. The ratio b_(ld)/b_(sd) of the width b_(id) of the Lavalnozzle 3 to the width b_(sd) of the molten material nozzle 4 is here >1and <10.

The production of fine powder by atomizing molten material with gasespursuant to the method is described in the following examples:

EXAMPLE 1

Tin solder molten material Sn62Pb36Ag2 having a temperature of 400° C.is issued from a graphite molten material nozzle having a rectangulardischarge cross-sectional area of 15 mm² with a length of 30 mm and adiameter of 0.5 mm. The Laval nozzle that is used has at its narrowestcross-sectional area a length of 33 mm and a thickness of 3.0 mm. Theatomizing gas is nitrogen having an overpressure p₁ of 20 bar overambient pressure. In the gas chamber 2, the so-called atomizing tower,there is also disposed nitrogen having an overpressure p₂ of 0.1 bar.The atomization takes place at a molten material throughput of 143 g/scorresponding to 8.6 kg/min=516 kg/h at a specific gas consumption of2.8 kg nitrogen (N₂) per kg metal. The achieved average granulardiameter of the produced powder is 9 μm.

EXAMPLE 2

Molten steel material of the alloy 42 Cr Mo 4, material number 1.7225,at a temperature of 1750° C., is issued out of a zircon dioxide moltenmaterial nozzle having a substantially rectangular cross-sectionalopening of 35 mm², a length of 50 mm, and a diameter of 0.7 mm. TheLaval nozzle, at its narrowest cross-sectional area, has a length of 55mm and a thickness of 3.5 mm. The atomizing gas is argon having anoverpressure p₁ of 30 bar over ambient pressure. Disposed in theatomizing tower 2 is again nitrogen having an over pressure p₂ of 0.1bar. The atomization is effected at a molten material throughput of 333g/s corresponding to 20 kg/min, corresponding to 1200 kg/h at a specificgas consumption of 4.5 kg argon (Ar) per kg metal. An average granulardiameter of the manufacturing powder of 9.5 μm was achieved.

EXAMPLE 3

A silver molten material having a temperature of 1060° C. issued out ofa graphite molten material nozzle having a substantially rectangulardischarge cross-sectional area of 20 mm² at a length 20 mm and adiameter of 1.0 mm. The Laval nozzle at its narrowest cross-sectionalarea had a length of 24 mm and a thickness of 4.0 mm. The atomizing gaswas nitrogen (N₂) having an overpressure p₁ of 18 bar over ambientpressure. In the atomizing tower 2 there was again disposed nitrogen(N₂) having an overpressure p₂ of 0.1 bar. The atomization was effectedat a molten material throughput of 233 g/s corresponding to 14 kg/min,corresponding to 840 kg/h at a specific gas consumption of 1.67 kgnitrogen (N₂) per kg metal. An average granular diameter of 9.0 μm wasachieved.

EXAMPLE 4

An aluminum molten material having a temperature of 800° C. was issuedfrom an alumina molten material nozzle (Al₂ O₃) having a substantiallyrectangular discharge cross-sectional area of 120 mm² at a length of 200mm and a diameter of 0.6 mm. The Laval nozzle, at its narrowestcross-sectional area, has a length of 205 mm and a thickness of 3.0 mm.The atomizing gas is a mixture of nitrogen and oxygen having an oxygencontent of 1% with an overpressure p₁ of 30 bar over ambient pressure.In the atomizing tower 2 there is again disposed the nitrogen/oxygenmixture having an overpressure p₂ of 0.2 bar, whereby small quantitiesof the oxygen react with aluminum particles on the surface and form athin, stable oxide layer. The atomization is effected at a moltenmaterial throughput of 785 g/s corresponding to 74.1 kg/min,corresponding to 2826 kg/h at a specific gas consumption of 5.9 kgnitrogen (N₂) per kg metal. An average granular diameter of 10.1 μm wasachieved.

EXAMPLE 5

A potassium chloride molten material having a temperature of 820° C. wasissued out of a graphite molten material nozzle having a substantiallyrectangular discharge cross-sectional area of 30 mm² at a length of 30mm and a diameter of 1.0 mm. The Laval nozzle has at its narrowestcross-sectional area a length of 33 mm and a thickness of 3.5 mm. Theatomizing gas is air having an overpressure p₁ of 20 bar over ambientpressure. In the atomizing tower 2 there is again disposed air having anoverpressure p₂ of 0.1 bar. The atomization is effected at a moltenmaterial throughput of 220 g/s corresponding to 13.2 kg/min,corresponding to 792 kg/h at a specific gas consumption of 22.1 kg airper kg salt. An average granular diameter of 8.5 μm was achieved.

EXAMPLE 6

A polyethylene molten material (LDPE), at a temperature of 175° C., isissued out of a stainless steel molten material nozzle having arectangular discharge cross-sectional area of 15 mm² at a length of 30mm and a diameter of 0.5 mm. The Laval nozzle, at its narrowestcross-sectional area has a length of 33 mm and a thickness of 3.0 mm.The atomizing gas is nitrogen (N₂) having an overpressure p₁ of 10 barover ambient pressure. In the atomizing tower 2 there is again disposednitrogen (N₂) having an overpressure p₁ of 0.1 bar. The atomization iseffected at a molten material throughput of 20 g/s corresponding to 1.2kg/min, corresponding to 72 kg/h at a specific gas consumption of 9.1 kgnitrogen (N₂) per kg polymer. An average granular diameter of 20 μm wasachieved.

The specification incorporates by reference the disclosure of Germanpriority documents DE 197 58 111.0 of Dec. 17, 1997 and German PatentApplication priority document PCT/EP98/08180 of Dec. 14, 1998.

The present invention is, of course, in no way restricted to thespecification disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

List of Reference Numerals

1. Gas chamber having pressure p₁.

2. Gas chamber having pressure p₂.

3. Laval nozzle.

4. Molten material nozzle.

5. Molten material.

6. Film of molten material.

7. Stream of particles.

p₁ pressure above the Laval nozzle.

p₂ pressure above the Laval nozzle.

a_(sd) length of the molten material nozzle.

b_(sd) width of the molten material nozzle.

a_(id) length of the Laval nozzle.

b_(id) width of the Laval nozzle.

What is claimed is:
 1. A method of producing fine powder, by atomizingmolten material with gas, including the steps of: causing moltenmaterial to flow, in a form of a film, out of a molten material nozzlehaving an essentially rectangular discharge cross-sectional area; andthereafter causing said molten material, together with an atomizing gas,to flow through an initially converging and then diverging gas nozzlethat is in the form of a linear Laval nozzle, has an essentiallyrectangular cross-sectional area, and through which flow is laminar,whereby a laminar accelerated gas flow stabilizes and simultaneouslystretches said film of molten material in the converging portion of saidLaval nozzle until said film of molten material, after passing anarrowest cross-sectional area of said Laval nozzle, is uniformlyatomized over an entire length thereof.
 2. A method according to claim1, wherein a ratio of pressure above said Laval nozzle to pressure belowsaid Laval nozzle is set at least to a critical pressure relationship ofthe atomizing gas that is utilized such that in said narrowestcross-sectional area of the Laval nozzle said atomizing gas reaches thespeed of sound.
 3. A method according to claim 2, wherein said pressureratio is set to a value >2, preferably >10.
 4. A method according toclaim 1, which includes the step of preheating said atomizing gas.
 5. Amethod according to claim 1, which includes the step of heating moltenmaterial that issues from the molten material nozzle by means ofradiation.
 6. A method according to claim 1, which includes the step ofalso atomizing contaminated or impure molten material by said moltenmaterial nozzle.
 7. A method according to claim 1, which includes thestep of using a metal, a metal alloy, a salt, a salt mixture or asynthetic material that can melt as molten material that is to beatomized.
 8. A method according to claim 1, which includes the step ofusing an atomizing gas with which said molten material that is to beatomized does not react.
 9. A method according to claim 1, whichincludes the step of using an atomizing gas with which said moltenmaterial that is to be atomized reacts entirely or partially.
 10. Anozzle for atomizing molten material with gas for producing fine powder,comprising: a molten material nozzle having an essentially rectangulardischarge cross-sectional area; and a gas nozzle disposed downstream ofsaid molten material nozzle below said discharge cross-sectionalthereof, wherein said gas nozzle is in the form of a Laval nozzle andalso has an essentially rectangular cross-sectional area, wherein saidgas nozzle generates an initially converging laminar accelerated gasflow in a converging portion thereof that stabilizes and simultaneouslystretches a film of molten material flowing there through, and whereinsaid film of molten material, after passing a narrowest cross-sectionalarea of said gas nozzle, is uniformly atomized over its entire length ina diverging portion of said gas nozzle.
 11. A nozzle according to claim10, wherein a shortest sides of the essentially rectangularcross-sectional areas of at least one of said molten material nozzlematerial and said gas nozzle are formed by semicircular arches having adiameter that corresponds to a length of said short sides, so that anessentially rectangular cross-sectional area results.
 12. A nozzleaccording to claim 11, wherein a ratio of long sides of said rectangleto short sides of said rectangle of cross-sectional areas of at leastone of said molten material nozzle and said gas nozzle is at least >1,preferably >2, and especially preferably >10.
 13. A nozzle according toclaim 12, wherein a ratio of said gas nozzle to a width of said moltenmaterial nozzle is >1 and <100, preferably <10.
 14. A nozzle accordingto claim 11, wherein a length of said gas nozzle in said narrowestcross-sectional area thereof is greater than a length of said moltenmaterial nozzle.
 15. A nozzle according to claim 11, wherein alengthening of a long side of said molten material nozzle and acorresponding lengthening of a long side of said gas nozzle by the sameamount adapts a molten material throughput to a desired productioncapacity without altering a granular size of powder or increasing aspecific gas consumption.